A range of tennis rackets exist which have been designed to provide a more uniform, powerful, forgiving and controlled response to all hits, and a larger optimal (“sweet spot”) area of the racket face.
Typically, when a tennis player swings his racket to hit the ball, he assumes that the impact will be in the sweet spot of the racket face, and he swings accordingly. If the impact location on a conventional racket is not on the sweet spot, or off-center, the ball encounters strings of different length and the resultant ball trajectory is probably not going to be the desired one. Errors such as hits into the net, beyond the baseline, or too near the opponent, will often result. Further, balls hit off-center encounter strings of lengths that are different from one another and different from the lengths of the central strings. The ball will rebound with less speed than, and not consistent with, balls hit near the center of the racket face. Thus, for a given racket swing, the hit ball velocity is highly dependent on where on the face of the racket the ball is hit.
Additionally, with a conventional racket, when a ball strikes parallel strings of equal tension but of different lengths, the strings vibrate with different frequencies, so that the combined effect produces a less-than-optimal ball rebound speed.
In an effort to address some of the above deficiencies in conventional rackets, Woehrle et al., U.S. Pat. No. 4,834,383, disclosed a tennis racket having equal string lengths for a limited section of the racket. In order to achieve equal transversal string lengths over a limited section of the racket, the side central regions of the head of this racket are formed differently than conventional rackets. The side central regions have internal ridges forming flat inner faces and extending parallel to the axis. Longitudinal strings of equal length in this limited section are achieved by providing a reverse or crowned throat on the frame having a curvature identical to the opposite end or crown of the head. The racket was designed with equal string lengths in a limited section of the face of the racket so that, at least in this limited section, the strings would provide substantially the same response.
Further, although the Woehrle et al. patent suggested making the racket strings of equal length in the area of the sweet spot at least, a greater section of the racket face, such consideration was not reduced to practice, but was rejected as unmarketable and unusable. The Woehrle et al. patent stated that “no practical way of achieving this end has been found.”
Head, U.S. Pat. No. 3,999,756, suggested certain performance advantages provided by rackets with larger (elliptical) faces compared to the conventional rackets then in use. In particular, Head realized that the larger racket would move the racket face area in the beneficial direction toward the racket center of mass, would move the center of the racket face near the racket center of percussion, and would use longer strings which reduce the angular errors resulting from off-center hits.
Since Head's innovation, there have been numerous patents describing further improvements, including the Woehrle et al. patent. Although these developments led to further enhanced performance, the lack of serious computer modeling, as well as the lack of interesting new ideas, have impeded significant progress.